1. Field of the Invention
This invention relates to new isocyanate prepolymers containing ether and ester groups which are liquid at room temperature, to a process for their preparation and to their use as binders in solvent-free or low solvent polyurethane coating compositions, sealing compositions or casting compositions.
2. Description of the Prior Art
Coating compositions and casting compositions in which the binders are based on compounds which react to form polyurethanes are known (see e.g. Wagner Sarx, Lackkunstharze, 5th Edition, Carl Hanser Verlag, Munich, 1971, pages 153 to 173, and Kunststoff Handbuch, Volume 7, Hanser Verlag 1983, pages 540 to 561 and pages 425 to 428).
Solvent-free systems which enable coatings of any desired thickness to be produced in a single operating step are also described on pages 169 to 170 of the first mentioned publication and pages 556 to 559 of the second publication mentioned. The solvent-free coating compositions have become particularly important with the increasing interest in environmental hygiene. The preparation of solvent-free coatings requires the use of low viscosity starting materials.
Many binders containing isocyanate prepolymers and (potentially) isocyanate reactive compounds are known. Thus, for example, DE-OS 1,520,139 describes a process for the preparation of moisture hardening mixtures of polyisocyanates and polyketimines or polyaldimines in which the polyisocyanates used are preferably isocyanate prepolymers (NCO prepolymers). DE-AS 1,240,654 describes a process for the preparation of cross-linked synthetic resins from isocyanate prepolymers and special aromatic diamines. DE-OS 2,018,233 describes moisture hardening preparations of compounds containing isocyanate groups and polyoxazolidines.
The isocyanate prepolymers known in the art are prepared by the reaction of relatively high molecular weight polyhydroxyl compounds such as polyether or polyester polyols with excess quantities of di- or polyisocyanates.
Prepolymers based on polyester polyols give rise to synthetic resins with good resistance to light and aging by heat after they have been cross-linked but the prepolymers are highly viscous or even crystalline at room temperature due to their chemical constitution and therefore can only be worked up by the addition of solvents and/or plasticizers.
Prepolymers based on polyether polyols have a sufficiently low viscosity at room temperature to enable them to be worked up with little or no solvent. The synthetic resins obtained after cross-linking of these prepolymers have good hydrolysis resistance but insufficient resistance to light and oxygen. Another disadvantage is the increased formation of blisters which occurs during hardening of such prepolymers.
There are two solutions available according to the state of the art for combining the positive properties of polyester polyurethanes with those of polyether polyurethanes:
1. The preparation of prepolymers by reacting polyether/polyester mixtures with di- or polyisocyanates
2. The preparation of polyols containing ether and ester groups, e.g., as described in DE-OS 3,315,382, DE-PS 2,210,839, DE-OS 1,645,674, EP-A-0,178,562 and DE-OS 1,178,955, and the subsequent formation of prepolymers by reacting such polyols with di- or polyisocyanates.
The isocyanate prepolymers containing ether and ester groups prepared according to 1) or 2) are always more highly viscous than prepolymers based on polyether polyols. In many cases such prepolymers can only be used with the addition of organic solvents or plasticizers. It is well known, however, that the use of solvents is a disadvantage from an ecological point of view. Regulations demand the use of coating systems containing as little solvent as possible. The use of plasticizers also has disadvantages. Plasticizers remain in the coatings, are liable to impair the mechanical properties of the polymers and their adherence to the substrate and in the long term migrate out of the coatings.
An object of the present invention is to provide isocyanate prepolymers containing ether and ester groups which have low viscosities which are at least as low as isocyanate prepolymers based on known polyether polyols but which are free from their disadvantages.
This object has been achieved in accordance with the present invention with the isocyanate prepolymers described below.
The present invention is based on the surprising observation that prepolymers obtained from certain polyhydroxyl compounds containing ether and ester groups and commercially available polyisocyanates have markedly lower viscosities than prepolymers based on the same polyisocyanates and conventional polyethers and/or polyesters.